This application claims the priority of Japanese Patent Application No. 2002-010767 filed on Jan. 18, 2002, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical device in which a multilayer film is formed on a transparent substrate provided with a barrier layer, an optical system using this optical device, and a projector apparatus using the same. In particular, the present invention relates to an optical device, an optical system, and a projector apparatus which are suitable when the multilayer film functions as a dichroic film or the like.
2. Description of the Prior Art
In an illumination optical system/color-combining optical system of a color liquid crystal projector apparatus or the like, for example, a color-decomposing or -combining prism provided with a dichroic multilayer film for decomposing light source light into three colors or combining individual modulated light components from respective light valves has conventionally been in use.
Such a dichroic multilayer film is required to maintain a favorable spectral characteristic and a phase difference characteristic over the whole visible light wavelength region in a state irradiated with strong light source light.
On the other hand, a glass substrate formed with the dichroic multilayer film, i.e., prism, is required to have a low photoelastic coefficient and a low optical absorption index in order to maintain a favorable polarizing state, whereby a glass material containing a large amount of PbO and the like is used as its constituent material.
However, such a prism is continuously irradiated with strong light source light (light on the shorter wavelength side in particular), whereby molecules of Pb or the like contained in the prism are likely to diffuse into the dichroic multilayer film and may reach the uppermost layer thereof in some cases. In such a case, molecules of Pb or the like react with those of Ti or the like in TiO2, for example, constituting the dichroic multilayer film, thereby blackening TiO2 or the like, which may increase the optical absorption index of the dichroic multilayer film.
As measures against such a problem, a barrier layer may be disposed on the glass substrate so as to inhibit molecules from dispersing between the glass substrate and multilayer film.
Conventionally known as an optical device using a barrier layer is one in which a barrier layer made of MgF2 or SiO2 is interposed between a substrate and an antireflection multilayer film in order to inhibit reactions from occurring in a boundary between the substrate and the antireflection multilayer film, thereby suppressing optical absorption in the vicinity of the boundary Japanese Unexamined Patent Publication No. 2000-275402).
If the prior art mentioned above is employed for overcoming the problem mentioned above, the barrier layer may disorder the spectral characteristic and phase difference characteristic, whereby the product may become inferior in terms of performances as a color-decomposing or -combining prism or the like for a color liquid projector apparatus required to have a favorable spectral characteristic over the whole visible light region and a phase difference characteristic which can maintain a polarizing state.
If the thickness of the barrier layer in the above-mentioned prior art is adjusted such that the multilayer film and barrier layer attain favorable spectral and phase difference characteristics in total, the barrier layer will be too thin (e.g., several to ten-odd nanometers) to function as a barrier layer.
When a film is to be designed as a combination of the multilayer film and barrier layer, care must be taken so as not to affect functions of the multilayer film, whereby it becomes harder to design, thus taking a large amount of time and cost for development.
It is an object of the present invention to provide an optical device formed with a multilayer film and a barrier layer on the multilayer film side, which can attain favorable spectral and phase difference characteristics while favorably maintaining functions as the barrier layer, and save the time and cost for designing the film.
It is another object of the present invention to provide an optical system using such an optical device.
It is further object of the present invention to provide a projector apparatus using such an optical device.
The present invention provides an optical device comprising a light-transmitting optical substrate formed with a multilayer film and a barrier layer disposed on the multilayer film side thereof;
wherein the light-transmitting optical substrate has a refractive index Ns (at a reference wavelength of 632.8 nm) falling within the range of 1.7xe2x89xa6Nsxe2x89xa61.9; and
wherein the barrier layer has a refractive index Nb (at a reference wavelength of 632.8 nm) falling within the range of xe2x88x920.1xe2x89xa6Nbxe2x88x92Nsxe2x89xa60.1.
Preferably, in the optical device, the barrier layer is constituted by La2xAl2yO3(x+y), where 0.5xe2x89xa6x, and yxe2x89xa61.5.
Preferably, the barrier layer has a physical film thickness of at least 20 nm.
Preferably, the light-transmitting optical substrate is constituted by a glass material containing at least 50% by weight of at least one of PbO, Nb2O5, and TiO2.
Preferably, the value of Nbxe2x88x92Ns falls within the range of +0.05.
Preferably, the multilayer film comprises alternately laminated TiO2 and SiO2 layers.
The present invention provides an optical system comprising the optical device mentioned above.
Here, the multilayer film may be a dichroic film.
The present invention provides a projector apparatus comprising the optical device mentioned above.
Here, the multilayer film may be a dichroic film.